Saw someone else refer to gremlins in their question and it is what the client referred to it as, so hence the subject!
In any case, I was on-site at a client today setting up Ubiquity NanoStations point to point between buildings and swapping out a couple of switches. The NanoStations went in without much issue, but the switch swapping...that's another story.
We replaced a HP Procurve 1810G-8 (8-port gigabit) switch with a new HP ProCurve 1410-24G (24-port gigabit) switch. This was due to someone having tossed a 4-port Dlink router in as an extra switch and wanting to clean things up a bit. I then took the 8-port across the parking lot to the other building to use it to replace a Linksys standard 8-port 10/100 switch.
I unplugged the network cables, removed the old switch and power adapter, then setup the "new" HP 8-port switch. I plugged everything back in, all lit up but one... It would blink on for a moment, then go dark...30+ seconds go by...rinse repeat. Of course, this is the one that went to the downstairs switch which is what leads to the NanoStation and internet router. This connectivity has to be there, or the two users and two printers are unusable. Here's the first fun part - the downstairs switch is also a HP Procurve 1810G-8.
I did some basic troubleshooting and found that I had connectivity when I plugged a computer into the port upstairs and it allowed me to pull down manuals and what not from the internet. I connected to the switch, logged into it and checked for any specified settings that whoever was there before me might have done. It was generic. I also did this to the downstairs switch also.
I couldn't figure out why a computer would work, but a switch wouldn't. I hadn't seen something like this in years since switches started being auto-sensing. I even tried putting the old Linksys switch back and it did the same thing.
After much research and frustration, I got HP Support on the phone. He had me log into the switch and review things, and then illustrate what happened. We could see real time as the port came up then went down. He eventually had me pull the switch from upstairs, take it downstairs and connect it on the same ports with the same patch cable. The only difference being, it wasn't going through the cable in the wall.
It worked. No issue, it just came right up. We didn't change anything in the configs at all. The HP guy was also perplexed as to why a workstation would work fine, but a switch wouldn't. I've never seen that before. Has anyone seen this? What could be the cause? It would be preferred to not have to run a new cable to the location to get the connectivity we need back.
As it stands now, we have a cable running from two rooms away, connecting to the upstairs switch and all but one PC plugged into it and working. That one PC is using the "switch-hating" wall port quite happily.
Damn gremlins...